Lindsay and Halstead respond to an "officer down" call that leaves the team searching for a sniper who shot a young officer. Commander Crowley asks Voight to head up the manhunt city-wide. As the officer dies of his wounds at Chicago Med and public pressure mounts the team starts combing a long list of suspects.
A stolen vehicle is the first major lead but when the team checks it out another officer is shot in the head and is dead before she hits the ground. Lindsay, Atwater, Ruzek and Halstead all exchange gunfire with the sniper but can't catch him and deduce he must be military or with law enforcement training.
Lindsay discovers Robert Dean, an ex-Naval reservist on parole, has been vocal about his grudges with police. Dean is apprehended outside his home with assault weapons in the trunk of his car but he alibis out.
Halstead and Olinsky suggest that the sniper could be a former member of CPD and suspect Carl, the son of a cop killed in the line of duty who was kicked out of the police academy and left with a grudge. A comment from Carl's mother leads the team to his childhood home and Voight tries to talk the kid down.
But after Lindsay spots a bomb and two hostages inside the home Voight is forced to order Olinsky to take a sniper shot himself - killing Carl the same way he killed two cops. The team participates in a vigil for all their fallen officers, including Voight's father.
Meanwhle Sorensen, shell-shocked by having to carry the second dead officer to Chicago Med, refuses to go back out on duty and turns in his badge. That leaves Burgess free to finally get promoted to Intelligence. And Antonio tells his colleagues that he's leaving for the State's Attorney's Office.